Understanding Fan Laws in maintaining high performance of the Industrial Ventilation Systems.
The Fan Laws provide a fundamental understanding of how changing a fan’s speed affects its performance characteristics—flow rate, pressure, and power consumption. Integrating the Fan Laws into the discussion helps explain why adjusting fan speed based on the fan curve is essential in running any ventilation system meeting its objective while not wasting unnecessary energy. Please take note, the ability for the system performance meeting its objectives means to be able to meet adequate air changes if it is using dilution control, generating capture velocity within the effective range to remove airborne hazard at the point of generation, meeting the air flow rate for machinery vent as per the machineries operation manual and for any other application.
Fan Laws and Their Relevance
What are Fan Affinity Laws :- Fan Affinity Laws: Relationships between fan flow rate, speed, pressure and power used to calculate and predict additional points of operation from one fan curve to one or more additional fan curves by modulating the speed primarily.
1. First Fan Law: Airflow
(rpm2)/(rpm1) = (flow rate 2/ flow rate 1)
rpm2= ( flow rate 2 / flow rate 1 ) x rpm 1
• Airflow is directly proportional to the fan speed. Doubling the fan speed doubles the airflow.
Relevance:
By adjusting the speed, you can match the airflow requirement of the system to its actual demand without over- or under-ventilating.
However, at this point the demand of the air flow rate has to be based on the proper calculation of its demand such as effective calculation of additional LEV hood, increase of air change per hour due to additional emission load or changes in capture velocity as the initial basis used is not sufficient to remove the contaminant from site.
The upgrade here is based on additional demand of flow rate. The changes to be applied here is only the increase of fan impeller speed by other manipulating pulley sizes or modulation the frequency. No other modification is required as doing so will not fulfil the justification.
2. Second Fan Law: Pressure
(rpm2/rpm1) = (static pressure 2 / static pressure 1)^2
rpm2 = (static pressure 2 / static pressure 1)^2 x rpm 1
Static pressure varies with the square of the fan speed. Doubling the fan speed increases the pressure by a factor of four.
Relevance:
Deliverance of the flow rate demand has to undergo the resistance in the form of system static pressure. Then which is located at the downstream point of the exhaust or supply means will need to overall hood entry losses, plenum and slot losses, compound hood losses, friction losses, fitting losses, air cleaning device pressure losses, fan system effect and chimney jet cap losses. During the initial sizing of the fan, the system static pressure calculation has to be accurate. If it was calculated lower than the actual demand, the system flow rate will diminish. Similarly if it is overly estimated, the fan flow rate will be too high. If these were to happen, the fan speed modulations will be required as apart of testing and balancing.
In the event an upgrade is required due to expansion of additional hood or air changes or design basis enhancement as described above, the system static pressure must be calculated as to finalise the system design point. The vertical increase of the y axis in the fan curve which represents system static either in the form of static pressure or total pressure must be determined before the new fan speed can be determined by using the law above.
3. Third Fan Law: Power
(rpm2/rpm1) = (horse power 2 / horse power 1)^3
rpm2 = (horse power 2 / horse power 1)^3 x rpm 1
Power consumption varies with the cube of the fan speed. Doubling the speed increases power usage by a factor of eight.
Relevance:
Upgrade of fan speed using law 1 and law 2 is limited to the power the motor can deliver to reach the newly calculated speed. In the event the new speed requires a motor upgrade, this third fan law can be applied to meet the new system design point.
Similar to both fan law above, the flow rate and system static pressure calculation has to be very accurate. Once that is determine, upon completing the new fan speed simulation, the third law can be applied. However do calculate full load current (FLA) and ensure the wires and breakers used meets the new horse power’s full load current. The formula for FLA is as follows :-
FLA = (746 x HP) / ((3)^(1/3)x Voltage x Efficiency x Power Factor)
where :-
FLA denotes Full Load Current.
Voltage denotes name plate voltage.
Efficiency denotes the motor efficiency which can be found in name plate.
Power factor denotes the power factor of the motor can be found in name plate.
Why Adjust Fan Speed Based on Fan Laws and Fan Curves?
1. Meeting System Design Point - Air flow rate
• Using the First Fan Law, you can control the airflow rate by adjusting speed to meet the system design point as described above. The fan curve helps identify the required speed to achieve the desired airflow at a given system resistance and prevent under or oversizing. Understanding and applying this mechanics is very crucial in optimising energy consumption to reduce carbon emission. Excessive fan flow rate will require higher current to operate and increases cooling load in an enclosure.
2. Avoiding Static Pressure Mismatch
The Second Fan Law shows that pressure increases quadratically with speed. If the speed is too high, the pressure may exceed the system’s design point , leading to imbalance between air flow rate and static pressure. The common method to trouble shoot this excess air is to manipulate damper and increase the static pressure losses. The increase of pressure will reduce the excess flow rate. However this method is not good as the fan will be running at unnecessary high electrical load. Balancing the speed to meet the system design point is the best way to go by adopting the fan laws.
3. Optimizing Energy Efficiency
The Third Fan Law highlights how energy consumption increases rapidly with speed. A slight reduction in speed can yield significant energy savings while maintaining acceptable performance. Upsizing the motor of the current drive system must be done efficiently with the usage of the fan laws to ensure the right of amount of power is allocated for the drive system to churn the system design point required.
4. Dynamic System Adjustments
• Systems with variable demands usually in the case of future upgrading can be accomplished by using fan laws. The adjustment of the air flow rate and the system static pressure can be made to adjust the speed based on any new demand. This approach would enable the drive system to be sized up with the right full load current in advance for future usages. One just need to ensure the hood and duct sizing are done accordingly to ensure the system in balance the moment the upgrade are complimented to ensure the fan law compliments the individual flow rate used as the basis.
In summary, integrating Fan Laws into fan speed adjustments highlights the mathematical relationships governing performance. When paired with the fan curve, it enables precise, efficient, and safe fan operation across varying system demands. Fan laws are very crucial method to conduct Testing And Balancing exercise to run operation at the system design point or establish baseline point.
